1. Field of the Invention
This invention relates to a process for producing a toner for developing electrostatically charged images.
2. Description of the Prior Art
Heretofore, toners have been manufactured generally by fusion mixing of colorants and other additives into thermoplastic resins to disperse them uniformly therein, followed by pulverization and classification into desired particle sizes by means of a micropulverizing device and a classifier. This preparation method is capable of producing considerably excellent toners, but limited in certain respects. More specifically, in the process for producing toners by pulverization, the material must be fragile so as to be readily pulverized to a certain extent. However, if the material is too fragile, it may be excessively micropulverized, and therefore micropulverized portion of the particles thus obtained must be uneconomically removed in order to obtain a suitable distribution of particles, resulting in increased production cost of the toner. Moreover, such a toner may be sometimes further subject to micropulverization in copying machines. If a material with a low melting point or a pressure fixing material is employed to improve heat fixing characteristic of the toner, fusion of such a material may occur in the pulverizing device or the classifier, whereby continuous production of the toner may sometimes be rendered impossible. Other requisite conditions for toners are to have triboelectric charging characteristic suitable for development, to be capable of forming excellent images, to be stable in performance after storage, to be free from agglomeration such as blocking, to have a suitable heat or pressure fixing characteristic, to effect no contamination on a photosensitive member and so on.
Accordingly, in order to remove the drawbacks of the pulverization method, there have been proposed processes for producing toners through suspension polymerization. According to these processes, since no pulverization step is included, no fragility is necessary for the material and spherical shapes of the toner obtained are suitable for giving excellent free flowing property, and it is also possible to obtain toners excellent in heat or pressure fixing characteristic by controlling polymerization adequately, by use of a crosslinking agent, etc. or by incorporating some additive in the material.
However, it is technically difficult to conduct polymerization in a stable suspension system without any coalescence and to obtain fine polymer particles with a uniform distribution of particle diameters.
Accordingly, in order to prevent the polymer particles from coalescing as the polymerization proceeds, suspension stabilizers are used when the suspension polymerization of a polymerizable monomer system is carried out in water. The suspension stabilizers generally include finely divided, barely soluble inorganic compounds such as barely soluble salts, for example, BaSO.sub.4, CaSO.sub.4, MgCO.sub.3, BaCO.sub.3, CaCO.sub.3, and Ca.sub.3 (PO.sub.4).sub.2 ; inorganic polymers such as diatomaceous earth, silicic acid and talc; powders of metal oxides; and water-soluble polymers such as polyvinyl alcohol, gelatin, starches and the like.
Stirring also affects stability of polymerization and particle size. High speed stirring makes polymerization stable, but the size of particles obtained will be smaller than the desired size. On the contrary, low speed stirring may result in gelation of the reaction system, so that particles may not be formed. Therefore it is necessary to select suitable conditions.
However, even by these methods, it is difficult to obtain polymer particles with satisfactory sizes as toner, that is, minute particles with a number average size of about 10.mu.-20.mu.. This is because coalescence cannot sufficiently be prevented according to the methods of the prior art. Accordingly, there has been proposed a method using a combination of a polymerizable monomer and an inorganic dispersant which have mutually opposite charging polarities (Japanese Laid-open Patent Application No. 110945/1981). According to this method, the surfaces of polymerizable monomer particles are cationically (or anionically) charged by use of a cationic polymerizable monomer (or an anionic polymerizable monomer) or by addition of hardly soluble organic amine compounds, while the inorganic dispersant is anionically (or cationically) charged to the contrary, whereby the inorganic dispersant completely covers the surfaces of the polymerizable monomer particles uniformly by strong ionic bonding to prevent coalescence of the polymer so that fine particles with a number average size of about 10 to 20.mu. may be produced. However, the sizes of the particles produced according to these methods are not yet sufficiently satisfactory as toner. The distribution of particle sizes should desirably be as narrow as possible. In other words, if the particle sizes are more uniform, charges on individual particles will become substantially the same, so that images can be formed stably. As the distribution of particle sizes is made narrower, the images will be more stably formed, with improves reproducibility of fine lines without fog.
While not intended to be bound by any theory, sufficiently narrow particle size distribution cannot be obtained in the above-mentioned method, because cationic or anionic monomers, when added into a monomeric system, tend to gather at the surfaces of suspended particles, and slightly diffuse from the monomeric system to the dispersing medium whereby distinct interface between particles is not ensured and suspension of particles becomes slightly unstable.
Further, it is very difficult for the toner prepared according to the method as mentioned above to satisfy the antagonistic requirements of having good heat fixing characteristic and of having good anti-blocking characteristic as well as good developing characteristic. To improve heat fixing characteristic, it is needed to lower the melting temperature or glass transition temperature Tg of the polymerized resin, which, however, is disadvantageous to anti-blocking characteristic. To improve anti-blocking characteristic, at least Tg of the polymer must be raised up to a required non-blocking temperature or higher, higher Tg being still more advantageous.
However, if Tg of the toner prepared by this method is lowered to improve heat fixing characteristic by decreasing the molecular weight of the polymer, the anti-blocking characteristic will become worse because the toner is constituted of a substantially homogeneous polymer as a whole in spite of the presence of cationic (or anionic) groups gathering at the particle surfaces. This will in turn be reflected on development to worsen the quality of images; and vice versa, namely, if the polymer is made to have higher molecular weight or crosslinked to improve anti-blocking characteristic, heat fixing characteristic will become worse.